Polylactic acid (PLA) is expected as a base material for sustainable society in view of not only biodegradability thereof, but also, in these days, the characteristic that it can be produced by using biomass as a raw material.
Lactic acid monomers used as the raw material of polylactic acid are commercially produced by chemical synthesis or microbial fermentation. Although chemical synthesis of lactic acid always provides a racemic mixture, high optical purity L- or D-lactic acid can be produced by microbial fermentation depending on the selected microorganism. Optical purity of the raw material lactic acid greatly influences on the physical characteristics of polylactic acid. Poly-L-lactic acid (PLLA) formed by polymerization of only L-isomers, and poly-D-lactic acid (PDLA) formed by polymerization of only D-isomers have higher crystallinity compared with poly-DL-lactic acid (PDLLA), which is a random polymer of D-lactic acid and L-lactic acid, and also show higher heat resistance. Therefore, production of high optical purity lactic acid by fermentation is considered important.
Biomass raw materials used for various fermentative productions mainly consist of corn and sugarcane, and edible parts (starch, sucrose) of these are used. If non-edible biomass raw materials such as non-food crops, woody biomass and rice straw can be used for fermentative production, such production does not compete with food and feed production, and thus is desirable.
When non-edible biomass is used, not starch, but cellulose and hemicellulose constitute the main fermentation raw materials (carbon source). However, cellulose and hemicellulose cannot be directly used for lactic acid fermentation by a lactic acid bacterium, but require pretreatments (liquefaction and saccharification) for use in lactic acid fermentation.
Methods for the pretreatments can be roughly classified into physicochemical methods using acid, alkali, or the like, and biochemical methods using microorganisms or enzymes. However, as for the former type of methods, if it is intended to decompose cellulose into monosaccharides, treatments must be performed under relatively severe conditions, and fermentation inhibitors such as furfural are by-produced during such treatments. On the other hand, if the treatments are performed under relatively mild conditions, products of the treatments contains oligosaccharides derived from cellulose (cellooligosaccharides), and therefore there arises a disadvantage that, since cellooligosaccharides cannot be utilized by most of fermentative microorganisms, fermentation yield decreases. As for the latter biochemical methods, although research and development of enzymes themselves that decompose cellulose are advancing, enzymatic decomposition of cellulose into monosaccharides requires use of large amounts of various enzymes and long time. Furthermore, manufacturing cost of these cellulose decomposition enzymes is extremely high, and glucose (C6 saccharide) and cellobiose (C6 disaccharide) as the hydrolysis products are also potent inhibitors of CBH and BGL (Non-patent documents 1 to 3).
Further, if hemicellulose is hydrolyzed, xylose (C5 saccharide) and oligosaccharides thereof (C5 oligosaccharides) are obtained, and if they are used for lactic acid fermentation, molar yield of lactic acid is usually halved compared with the case of using C6 saccharides (Non-patent documents 4 to 7).
As for lactic acid production from non-edible biomass, there are a plurality of reports concerning heterolactic acid fermentation and D-lactic acid fermentation (Non-patent documents 4 to 10). However, there are extremely few examples of fermentative production of L-lactic acid as the raw material of poly-L-lactic acid from mixed saccharides derived from non-edible biomass as the raw material, and any practically usable bacterium has not been found so far.